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Science · 2025 · 4 marks
CBSE 2025 · Region 5 · Set 1 · Q38
In human beings, there are 23 pairs of chromosomes. Out of these 23 pairs of chromosomes (i.e. 46 chromosomes), 22 pairs of chromosomes are called…
In human beings, there are $\displaystyle 23$ pairs of chromosomes. Out of these $\displaystyle 23$ pairs of chromosomes (i.e. $\displaystyle 46$ chromosomes), $\displaystyle 22$ pairs of chromosomes are called autosomes and one pair of chromosomes. i.e. two chromosomes are called sex chromosomes. The sex chromosomes are of two types - 'X' chromosomes and 'Y' chromosomes. The sex of a child (i.e. progeny), is decided at the time of fertilisation. In other words, at the time of zygote formation the sex chromosomes inherited from the parents of a child decide whether the new born will be a boy or a girl.
(a)
What are chromosomes ?
(b)
Why is the pair of sex chromosomes in human males called mismatched pair ?
(c)(A)
Show with the help of a flow chart that the statistical probability of getting a boy or a girl is $\displaystyle 50$ : 50.
OR
(c)
(B)
Mention two examples of animals where sex is not determined genetically like human beings. Describe in brief the method of sex determination in these animals.
Marking-scheme solution
(a) Chromosomes carry genes which control the traits of an organism./Chromosomes contain information for inheritance of features from parents to next generation in form of DNA (deoxyribonucleic acid) molecules
(b) Men have one normal sized X chromosome while Y chromosome is short.
(c) (A)
(B) • In a few reptiles; the temperature at which fertilized eggs are kept determine the sex of offspring.
In snails; the individual can change sex, indicating that is not genetically determined.
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CBSE Class 10 Science past-paper question from the 2025 board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.